[MODEL] Changed `update_document`, so attributes are fetched from `as_indexed_json`

Example of the problem:

    book = Book.create(title: "My book")
    book.title = "My book 2"

    book.save
    # Elasticsearch::Transport::Transport::Errors::BadRequest:
    #       [400] {"error":"MapperParsingException[failed to parse [title]]; nested: ElasticsearchIllegalArgumentException[unknown property [ru]]; ","status":400}

    book.instance_variable_get(:@__changed_attributes) # => {title: {ru: "My book 2"}}
    book.as_indexed_json # => {title: "My book 2}

Related: #166, #140
[MODEL] Clean up getting attributes from `as_indexed_json` in `update_document`

Basically, flipping the relationship here should be enough: select based on
`changed_attributes` keys from the `as_indexed_json` Hash, not vice versa.

Related: #166